A DFT and TD-DFT study on intermolecular charge transfer complexes of pyrene with phenothiazine and promazine

The ground state thermodynamic stabilities of intermolecular charge transfer complexes between acceptor molecule pyrene and donor molecules phenothiazine and promazine have been investigated with DFT methods. Their vertical singlet-singlet transitions have been calculated and characterized employing TD-DFT methods, as well. In the ground state, pyrene forms slightly stable intermolecular complexes with phenothiazine and promazine in the gas phase. The phenothiazine-pyrene, [phtz-pyr], and promazine-pyrene, [pmz-pyr], complexes are 3.76 and 2.75 kcal/mol more stable compared to their reactants, respectively. For both of the complexes, the most probable four transitions belong to the local excitations (LE) within donor or acceptor molecules. However, the S0 → S1 transitions with excitation wavelengths of ≈448 nm for [phtz-pyr] and ≈425 nm for [pmz-pyr] are pure CT transitions between the frontier molecular orbitals of donor and acceptor molecules. The S0 → S2 transitions with an excitation wavelength of ≈337 nm for [phtz-pyr] and ≈345 nm for [pmz-pyr] are also CT transitions and the charge transfer occurring in the [pmz-pyr] complex is an n → π∗ type while the character of CT transition in the [phtz-pyr] complex is purely π → π∗.